Perpendicular magnetic anisotropy in epitaxially strained cobalt-ferrite (001) thin filmsH. Yanagihara, Y. Utsumi, T. Niizeki, J. Inoue, and Eiji Kita

Citation: Journal of Applied Physics 115, 17A719 (2014); doi: 10.1063/1.4864048 View online: http://dx.doi.org/10.1063/1.4864048 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/115/17?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Anomalous thickness-dependent strain states and strain-tunable magnetization in Zn-doped ferrite epitaxial films J. Appl. Phys. 115, 173505 (2014); 10.1063/1.4874920 Erratum: “Extraordinarily large perpendicular magnetic anisotropy in epitaxially strained cobalt-ferriteCoxFe3xO4(001) (x=0.75, 1.0) thin films” [Appl. Phys. Lett. 103, 162407 (2013)] Appl. Phys. Lett. 104, 059902 (2014); 10.1063/1.4864102 Electron theory of perpendicular magnetic anisotropy of Co-ferrite thin films AIP Advances 4, 027111 (2014); 10.1063/1.4866279 Extraordinarily large perpendicular magnetic anisotropy in epitaxially strained cobalt-ferrite CoxFe3xO4(001) (x=0.75, 1.0) thin films Appl. Phys. Lett. 103, 162407 (2013); 10.1063/1.4824761 Perpendicular magnetic anisotropy in CoFe2O4(001) films epitaxially grown on MgO(001) J. Appl. Phys. 109, 07C122 (2011); 10.1063/1.3566079

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Perpendicular magnetic anisotropy in epitaxially strained cobalt-ferrite (001)thin films

H. Yanagihara,a) Y. Utsumi, T. Niizeki,b) J. Inoue, and Eiji KitaInstitute of Applied Physics, University of Tsukuba, Tsukuba 305-8573, Japan

(Presented 5 November 2013; received 23 September 2013; accepted 4 November 2013; published

online 7 February 2014)

We investigated the dependencies of both the magnetization characteristics and the perpendicular

magnetic anisotropy of CoxFe3–xO4(001) epitaxial films (x¼ 0.5 and 0.75) on the growth

conditions of the reactive magnetron sputtering process. Both saturation magnetization and the

magnetic uniaxial anisotropy constant Ku are strongly dependent on the reactive gas (O2) flow rate,

although there is little difference in the surface structures for all samples observed by reflection

high-energy electron diffraction. In addition, certain dead-layer-like regions were observed in the

initial stage of the film growth for all films. Our results suggest that the magnetic properties of

CoxFe3–xO4 epitaxial films are governed by the oxidation state and the film structure at the vicinity

of the interface. VC 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4864048]

I. INTRODUCTION

Magnetic anisotropy (MA) is one of the most important

properties of magnetic materials. In general, spin-orbit cou-

pling is a primary source of origin of MA; and therefore, a

large spin-orbit coupling constant and/or large orbital

moments are often observed in strong MA materials.1 It has

been known that bulk cobalt ferrite possesses an exclusively

large cubic MA of the order of �106 erg/cm3 among cubic

spinel ferrites, and further, it exhibits remarkably large val-

ues of the magnetostriction constants.2 Thin-films of

cobalt-ferrites are expected to exhibit large perpendicular

magnetic anisotropy (PMA) when substrate-induced lattice

distortions couple to the large magnetostriction.3,4 In the dis-

torted films, the Co2þ ions located at the B-sites of the spinel

structure probably acquire non-quenched orbital moments5

like that in the bulk.6–8 Again, MAs are mainly supposed to

originate from the considerably large orbital moments of the

Co2þ ions through the spin-orbit interaction. However,

in cobalt-ferrite thin-films magnetic properties, such as

MA and magnetization, are strongly dependent on the

film-preparation methods and conditions.5,9–13 Therefore, the

potential for spintronics/magnetics of the cobalt-ferrite films

is unknown and even the mechanism of the PMA is not clari-

fied yet. Thus, it is important to determine the optimum

growth conditions for the cobalt ferrite films to understand

the mechanism of PMA.

Although there are several reports on the magnetic prop-

erties of epitaxial cobalt-ferrite thin films grown by various

different methods, such as reactive molecular beam

epitaxy,10–12,14 pulsed laser deposition,9,13 and sputtering,15

most of the films grown by these methods exhibit remarkably

lower saturation magnetization (MS) than that of the bulk

material. We recently determined that reactive sputtering

appears to be an appropriate method to prepare spinel-type

iron oxide (Fe3–dO4) films such as Fe3O4(001) and

c–Fe2O3(001) grown on MgO (001) substrates. The MS val-

ues of the Fe3O4(001) and c–Fe2O3(001) films are compara-

ble with that of the bulk material. Based on our previous

knowledge regarding Fe3–dO4 film growth, we have also

found that CoxFe3–xO4 epitaxial films with x¼ 0.75 grown

on MgO(001) exhibit strong uniaxial PMA of the order of 10

Merg/cm3 and large MS values comparable with that of bulk

material.16

In general, optimizing the growth conditions for reactive

magnetron sputtering even with a single alloy target is compli-

cated. These conditions include (i) growth temperature (sub-

strate temperature), (ii) sputtering power, (iii) process pressure,

and (iv) oxygen-gas flow rate among others. Since the number

of parameters concerning sputtering conditions is so large, the

growth conditions cannot be easily optimized. The optimized

growth conditions are also possibly dependent on the composi-

tion of cobalt ferrites. In this paper, we report on the magnetic

properties of cobalt ferrite epitaxial films with two different

compositions grown on MgO (001) substrates under different

conditions as a function of oxygen pressure. The results of our

study provided two significant findings. First, the magnetic

properties of CoxFe3–xO4 films are dependent on the oxygen

pressure during the sputtering process. Second, the CoxFe3–xO4

films initially exhibit considerably lower magnetization values

even under optimal conditions.

II. EXPERIMENTAL

Thin films of two different composition ratios of

CoxFe3–xO4(001) (x¼ 0.5 and 0.75) were grown epitaxially

on cleaved MgO (001) substrates via the reactive rf magne-

tron sputtering technique using 2-in.-diameter Co-Fe alloy

targets with the desired compositions of Co0.17Fe0.83 and

Co0.25Fe0.75. Prior to film growth, MgO (001) substrates

were annealed at 400 �C for 30 min. The process gas used

was a mixture of Ar and O2. The flow rate of Ar was fixed at

20.0 sccm corresponding to a partial pressure of �0.5 Pa,

a)Electronic mail: yanagiha@bk.tsukuba.ac.jp.b)Present address: Advanced Institute for Materials Research, Tohoku

University, 2-1-1 Katahira, Sendai 980-8577, Japan. Electronic mail:

t-niizeki@imr.tohoku.ac.jp.

0021-8979/2014/115(17)/17A719/3/$30.00 VC 2014 AIP Publishing LLC115, 17A719-1

JOURNAL OF APPLIED PHYSICS 115, 17A719 (2014)

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whereas that of O2 (/O2) was varied from 4.0 to 10.0 sccm

as a growth parameter. Under this condition, the O2 pressure

increases by �0.1 Pa during the sputtering process. The

growth temperature was maintained at 300 �C and the

rf-power of the sputtering process was set at 100 W. The

other details of the growth conditions can be found in our

previous work on epitaxial Fe3–dO4 films.17 The surface

structures were observed by reflection high-energy electron

diffraction (RHEED) immediately after CoxFe3–xO4 growth.

The film thicknesses were determined via the x-ray reflectiv-

ity (XRR) technique using a conventional h=2h diffractome-

ter. All the magnetic measurements were carried out at room

temperature (RT). The magnetization was measured using a

superconducting quantum interference device, SQUID-VSM

(Quantum Design, MPMS) at fields up to 670 kOe. The

MAs were evaluated from the magnetic torque curves meas-

ured over the range of 0–90 kOe by a torque magnetometer

(Quantum Design, PPMS Tq-Mag). The typical sample

dimension was less than 2.0� 2.0 mm2 and the samples

were shaped as squares.

III. RESULTS AND DISCUSSION

Figure 1 shows the typical RHEED images of the MgO

(001) substrate and 40-nm-thick CoxFe3–xO4(001) (x¼ 0.5)

film grown under the condition of /O2¼ 7:0 sccm at a sub-

strate temperature of 300 �C. Clear streaks and Kikuchi lines

can be observed in both surface images, indicating that the

surface is sufficiently flat and the crystalline quality of the

film is good. In the image corresponding to CoxFe3–xO4,

there are additional streaks between those corresponding to

MgO (001), indicating that the lattice unit of CoxFe3–xO4 is

approximately twice the lattice-unit size of MgO. Here, we

remark that our x-ray-diffraction analysis revealed that the

CoxFe3–xO4 films were coherently grown on MgO (001) sub-

strates with an in-plane lattice constant that was just twice

that of MgO.16 The crystal lattice of the CoxFe3–xO4 films

grown on MgO(001) is definitely strained with in-plane

stretched stress.

The results of the typical magnetization measurements

at RT are shown in Fig. 2. Finite remanences are observed in

the MH-loops under the application of an out-of-plane mag-

netic field, while no remanence appears in the in-plane MH-

loops, indicating that the CoxFe3–xO4(001) films grown on

MgO(001) are perpendicularly magnetized films. Since the

MS values of most of the previously reported CoxFe3–xO4

films are significantly less than that of the bulk material, the

MS value can be a good criterion to optimize the growth con-

dition. In fact, MS is strongly dependent on /O2for all com-

position ratios, and some of the samples exhibit MS values

close to the bulk value of CoxFe3–xO4.

In order to examine the thickness dependence of the MS

values of the CoxFe3–xO4 films, the product of MS and thick-

ness t was plotted against t (see Fig. 3). Each value of MStwas obtained by dividing the measured magnetic moment by

the area of the film. The thickness values were determined

by the oscillation periods observed in the XRR results. In

this plot, the slopes correspond to the MS values, and the

horizontal-axis intercept suggests the existence of a deadlayer. From Fig. 3, we note that most of the CoxFe3–xO4 films

show a linear behavior, meaning that the MS values deter-

mined in this manner are reasonably accurate. Although the

MS values for x ¼ 0:75 appears dependent on the /O2that

for x ¼ 0:5 is almost constant with respect to /O2. It is to be

noted that there are finite horizontal-axis intercepts for all

samples with different values of x and /O2. The typical value

of the intercepts approximately ranges as 2-3 nm, suggesting

that the dead layer or the region of smaller MS exists in the

initial growth stage. When spinel ferrite films are grown on

the surfaces of non-spinel structures, such as MgO or SrTiO3

FIG. 1. RHEED images of (a) MgO(001), and (b) CoxFe3–xO4 (x ¼ 0:5)

samples in [100] azimuth.

FIG. 2. Out-of-plane MH-loops for CoxFe3–xO4 films with x ¼ 0:5 and

x ¼ 0:75. The film thicknesses for x ¼ 0:5 and x ¼ 0:75 are 50 nm and

49 nm, respectively.

FIG. 3. Thickness dependence of MS for (a) x ¼ 0:5 and (b) 0.75. The verti-

cal and horizontal axes correspond to the CoxFe3–xO4 film thickness t and

the product of the saturation magnetization and the thickness (MSt). Straight

lines indicate the results of linear regression.

17A719-2 Yanagihara et al. J. Appl. Phys. 115, 17A719 (2014)

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substrates, a considerable number of anti-phase boundaries

(APBs) emerge, as reported for Fe3O4 (001) films grown on

a MgO (001) substrate. The formation of high-density APBs

results in a corresponding reduction in MS and/or high-field

finite susceptibility.18

Figure 4 shows the /O2dependence of the MS and Ku

values of CoxFe3–xO4(001) for different values of x. For

x ¼ 0:5, the trend of MSð/O2Þ exhibits a maximum at around

/O2¼ 8:0 sccm, and the maximum MS is approximately

430 emu/cm3 that is comparable with the bulk value. On the

other hand, MSð/O2Þ reaches a maximum of �400 emu/cm3

at around /O2¼ 6:0 sccm for x ¼ 0:75. Moreover, the trend

indicates that the larger-MS samples exhibit larger Ku.

Here, we consider how /O2affects the magnetic proper-

ties of CoxFe3–xO4 films. According to our previous report

on the epitaxial film growth of Fe3–dO4 (0 � d � 13) by using

the reactive magnetron sputtering technique, /O2directly

affects the sputtering mode.17 We noticed that the presence

of a threshold /O2value determining whether the sputtering

process was the metal or oxide mode. In addition, depending

on the sputtering mode, Fe3O4 and c–Fe2O3 films can be

selectively grown; in other words, the ratios of Fe2þ and

Fe3þ ions are controlled by /O2. Similar to the case of

Fe3–dO4 films, the valences of both Fe and Co ions in the

CoxFe3–xO4 films may possibly undergo changes via varia-

tion of /O2, resulting in the observed MS values of

CoxFe3–xO4 films.

Finally, we briefly comment on the relation between MS

and x. As shown above, MS equals the saturation magnetiza-

tion value of bulk CoxFe3–xO4 for x ¼ 0:5 and 0.75 upon

carefully tuning the /O2. Although experimental results for

CoxFe3–xO4 films with x¼ 1 are not presented in this paper,

it appears fairly difficult to prepare a CoFe2O4 film with an

MS comparable with that of the bulk material in our attempts.

Assuming that the valencies of Fe and Co are 3þ and 2þ,

respectively, for the sample corresponding to x ¼ 1 and that

the film also possesses an inverse spinel structure, the possi-

ble combinations of the B-site occupation by the same num-

ber of Fe3þ and Co2þ ions is limited, in the same manner as

Anderson’s criterion for Verwey ordering in Fe3O4.19

Therefore, it could be difficult for the Fe3þ and Co2þ ions to

migrate between the B-sites and occupy optimized positions

within a limited interval of time. As a result the samples

with x� 1 possibly contain a large number of various

defects. Although this is no more than a speculation, in order

to grow the CoxFe3–xO4 films with x¼ 1 by the sputtering

technique, we speculate that tuning of the other parameters

pertaining to the growth conditions may be required.

IV. SUMMARY

We investigated the dependencies of both the magnet-

ization characteristics and the perpendicular magnetic anisot-

ropy of CoxFe3–xO4 (001) epitaxial films (x¼ 0.5 and 0.75)

on the O2 flow rate in the reactive magnetron sputtering pro-

cess. The thickness-dependent magnetization measurements

revealed the existence of dead-layer regions in the initial

stage of the film growth for all films. Our results suggest that

the magnetic properties of CoxFe3–xO4 epitaxial films are

governed by the oxidation state and the film structure during

initial growth.

ACKNOWLEDGMENTS

This work was supported by the Elements Science and

Technology Project of MEXT, Japan. The MH-loop meas-

urements were carried out at NIMS, Tsukuba.

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FIG. 4. Oxygen flow-rate dependence of MS (circles) and Ku (diamonds) of

CoxFe3–xO4 with different composition ratios ðx ¼ 0:5; 0:75Þ. Broken lines

serve as visual guides.

17A719-3 Yanagihara et al. J. Appl. Phys. 115, 17A719 (2014)

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